Current behavioral medicine research investigates the role of behavioral stress in the development of cardiovascular disease. Implicit in the rationale for much of this psychophysiological research are assumptions that individuals differ in characteristic reactivity to stressful situations at home and at work, and that these differences can be identified reliably by observing individual differences in cardiovascular reactivity to challenging laboratory tasks. The studies proposed here will test these assumptions by comparing individual differences in heart rate (HR) reactivity during challenging laboratory tasks and 24-hour ambulatory HR reactivity. The results should provide needed support for assumptions underlying important current psychophysiological research. Study 1 will examine the extent to which individual differences in HR reactivity to challenging laboratory tasks generalize to ambulatory HR during usual daily life. Each of 72 healthy young men will complete one laboratory session, during which they will sit quietly and then perform three challenging psychomotor or cognitive tasks; one 24-hour ambulatory recording day, during which HR and physical activity will be recorded minute-by-minute; and one graded maximal exercise test. The main hypotheses are that, compared to nonreactors, laboratory HR reactors will show greater mean and maximum daytime ambulatory HR and greater daytime ambulatory HR variability, but similar basal and minimum daytime ambulatory HR. Measures of functional aerobic capacity derived from the exercise test and of ambulatory physical activity and emotion will be used to investigate contributions of these factors to obtained relationships between laboratory and ambulatory HR. Study 2 examines the stability over one year of individual differences in laboratory HR reactivity and ambulatory HR variability. Study 2 subjects will be recruited one year later to repeat laboratory, ambulatory and exercise procedures identical to those of Study 1. Study 3 will test the hypotheses that ambulatory HR will be higher on days of relatively high life stress or challenge, and that this difference will be greatest for laboratory HR reactors compared to nonreactors. Each of 36 medical students will complete one laboratory session, two ambulatory recording days, and one graded exercise test. One ambulatory recording day will be a day of relatively low challenge (going to class); the other will be a day of relatively high challenge (patient care practicum). Laboratory, ambulatory, and exercise test procedures will be identical to those of Study l.